It is often necessary to expose an image from a transparency onto a light sensitive receiving element. Such receiving element can be a printed circuit board, a conducting foil, elements used in etching, etc. In practice, such elements are brought into contact with the image bearing transparency in a printing frame or similar device and exposed to a special light source. In the case of printed circuit boards, the boards are generally preworked to have predrilled holes in a predetermined arrangement. A difficulty arises in assuring good registration, i.e., an accurate fit, between the transparency and the board, especially when several boards are to be consecutively exposed. The problem is compounded in cases involving two-sided printed circuit boards. In these cases, the exposure takes place, as a rule, simultaneously from both sides using two transparencies; hence, good registration must be obtained on both sides of the board.
In order to assure such good registration, the practice has been to visually align the transparency (commonly referred to in the trade as photomask) with the printed circuit board and to secure it to the board with adhesive tape. This method, however, is not very accurate and is somewhat dependent of operator's skill. In addition, this method is cumbersome and productivity is low.
In an effort to partially eliminate the deficiencies of this visual alignment method, holding pins have been used to hold the photomask and board together through alignment holes in both. In principle at least, the desired reproducibility of alignment when replacing boards can be obtained by this method provided, of course, that the alignment holes are located in both the printed circuit board and the photomask with sufficient accuracy so that the pattern to be reproduced remains in registration with the punched holes in the board. The disadvantage of this method is, however, that once punched, the photomask can no longer be adjusted in relation to the board, and therefore, any misalignment due to punching and manufacturing tolerances between the printed circuit board and the transparency cannot be corrected. This disadvantage results in a high number of rejections or refinishing operations.
German patent application DT-OS No. 2,330,726 discloses a device for exposing single and double sided printed circuit boards. The device comprises a printing frame which includes means for mounting a transparency. These means in turn comprise a working surface within a frame on which a transparency may be adhesively affixed and which working surface can be moved in two directions in one plane by means of adjusting screws on the mounting frame. Holes through the working surface allow alignment pins to be passed through to center the printed circuit board to be exposed over the transparency taped on the work surface. Special alignment holes therefore must be formed on the bottom work surface as well as the printed circuit board to permit insertion of the alignment pins. The disadvantage of this method is that evacuation of the printing frame is more complicated and every time a new transparency is used, it must be realigned and retaped on the work surface. In addition, the thumb screws used to move the work surface which holds the transparency is cumbersome. Since the aligning holes on the printed circuit are not absolutely tolerance free, the board can shift some, thus destroying an initially well aligned arrangement.
In this device, therefore, a trial exposure is recommended before actual exposure. Using this device, it is difficult to quickly and accurately compensate for manufacturing tolerances on the boards, especially if after each subsequent alignment, one needs one or more trial exposures. Another disadvantage to this method is that whenever the size of the printed circuit board is changed, the bottom work surface must be punched again to permit passage of the holding pins at the new location. The result is that the bottom work surface must be provided with a large number of perforations which are always present within the area that will be exposed. These create local variations in the exposure, especially in cases where a change is made from small printed circuit boards to printed circuit boards of a larger format.